Power transformers are considered prime power equipment in an electrical power system, and faults in power transformers may lead to major consequences, both in terms of power failure for large groups of customers and in the terms of cost/time associated with the repair or replacement of a transformer. Major power equipment has been protected against faults with use of protection devices. These devices detect various fault conditions in the power system and provide trip signals to circuit breakers associated with power equipment to isolate the faulty equipment or the protected power equipment from the system.
In the present disclosure, a protection device may also be referred to as an intelligent electronic device (IED), protection relay or simply as a relay. One of the protection devices for power equipment is a differential relay. Known power transformer differential relays can utilize individual phase currents from different windings of the transformer in order to form the phase-based differential currents to monitor faults in the power transformer or the power system and protect the power transformer. The individual phase currents from different windings can be measured with the help of a current transformer (CT) connected in the path of each winding of the power transformer.
An exemplary technique for detecting abnormal operating conditions, such as unbalanced conditions in a power transformer (for example, applicable for any three phase equipment and transmission system) and resulting faults due to unbalanced operation, is to use phase sequence information (for example negative sequence currents). To provide protection, the protection relay should provide a trip signal to its respective circuit breaker before any damage or degradation can occur in the power equipment. Performance within one AC cycle time (power cycle) is, for example, desired.
Failure in a CT (for example, CT secondary winding failure by breakage or winding disconnections), can cause malfunction of the protective relays resulting in false tripping (spurious operation). Thus, the detection of an abnormal operating condition in the system may not be authentic.
Further, in modern numerical multifunction protection relays, CT secondary failure can also cause malfunctioning of other protection functions (such as a broken primary conductor protection) or can pose a safety hazard because of high voltage build up at the terminal as a result of breakage in a CT secondary winding/connection.
Detection of an authentic abnormal operating condition (e.g., faults) for transformers can be carried out by additionally monitoring other parameters or derivable values from these additional measurements (e.g., current/voltage) made in the system. Some examples are use of voltage values as a reference together with the monitored electrical current values, use of current values from another CT as a reference, and so forth. These arrangements involve an additional channel in the IED to supply a reference voltage or current to the IED.
An example is provided to illustrate the difficulty in supplying a reference voltage or current signal to the protection relay. Here, the CT secondary supervision is performed with use of a separate reference current input but the reference CT input would be needed for each CT set separately, thus making it a rather expensive solution and more difficult to integrate into the protective relay.
In a three winding differential protection relay, 12 CT inputs would be used to cover both the protection specifications (9CT) and the CT secondary supervision specifications (3CT) at the same time. Some known solutions use voltage as a reference for the current in detection of CT secondary failure, but also this solution can involve some extra measurements on top of the differential protection itself.